1. Technical Field
The present invention relates to a technique of driving a discharge lamp that lights by discharge between electrodes.
2. Related Art
A high-intensity discharge lamp, such as a high-pressure gas discharge lamp, is used as a light source for an image display device, such as a projector. As a method of making the high-intensity discharge lamp light, an alternating current (AC lamp current) is supplied to the high-intensity discharge lamp. Thus, in order to improve the stability of light arc occurring within a high-intensity discharge lamp when supplying an AC lamp current to make the high-intensity discharge lamp light, JP-T-2004-525496 proposes to supply to the high-intensity discharge lamp an AC lamp current which has an almost constant absolute value and of which a pulse width ratio between a pulse width of a positive pulse and a pulse width of a negative pulse is modulated.
However, even if the high-intensity discharge lamp is made to light by performing pulse width modulation of the AC lamp current, a distance between discharge electrodes increases as the discharge electrodes wear away. Then, a voltage (lamp voltage) between the discharge electrodes rises. Thus, if the lamp voltage rises, it becomes difficult to maintain projections formed at the tips of the discharge electrodes in order to stabilize the light arc. As a result, lighting of the high-intensity discharge lamp becomes difficult. This problem is not limited to the high-intensity discharge lamp but is common in various kinds of discharge lamps that emit light by arc discharge between electrodes.